1. Field of the Invention
The present invention relates to a control unit for an electric power steering apparatus that provides steering assist force by a motor to a steering system of an automobile or a vehicle. The present invention particularly relates to a control unit for an electric power steering apparatus of which steering performance has been improved by arranging such that a steering return control recognizes a turn and a return of a steering wheel and information on a steering speed is used for calculating a current for controlling the return of the steering wheel.
2. Description of the Related Art
An electric power steering apparatus that applies assist load to the steering apparatus of an automobile or a vehicle with turning effort of a motor applies the driving force of the motor to a steering shaft or a rack axis based on a transmission mechanism like gears or belts via a reduction gear. Such a conventional electric power steering apparatus carries out a feedback control of a motor current for accurately generating an assist torque (a steering assist torque). The feedback control is for adjusting a motor application voltage so as to minimize a difference between a current control value and a motor current detection value. The motor application voltage is generally adjusted based on a duty ratio of a PWM (pulse width modulation) control.
A general structure of the electric power steering apparatus will be explained below with reference to FIG. 1. A shaft 2 of a steering wheel 1 is connected to a tie rod 6 of running wheels through reduction gears 3, universal joints 4a and 4b and a pinion rack mechanism 5. The shaft 2 is provided with a steering angle sensor 7 for detecting a steering angle of the steering wheel 1, and a torque sensor 10 for detecting a steering torque. A motor 20 for assisting the steering force of the steering wheel 1 is connected to the shaft 2 through the reduction gears 3. A control unit 30 for controlling the power steering apparatus is supplied with power from a battery 14 through an ignition key 11. The control unit 30 calculates a steering assist command value I of an assist command based on a steering torque T detected by the torque sensor 10, a vehicle speed V detected by a vehicle speed sensor 12, and a steering angle xcex8 detected by the steering angle sensor 7. The control unit 30 then controls a current to be supplied to the motor 20 based on the calculated steering assist command value I.
The control unit 30 is mainly composed of a CPU. FIG. 2 shows general functions to be executed based on a program inside the CPU.
Functions and operation of the control unit 30 will be explained below. A steering torque T detected by the torque sensor 10 and then inputted is phase-compensated by a phase compensator 31 for increasing the stability of the steering system. The phase-compensated steering torque TA is inputted to a steering assist command value calculator 32. A vehicle speed V detected by the vehicle speed sensor 12 is also inputted to the steering assist command value calculator 32. The steering assist command value calculator 32 calculates a steering assist command value I as a control target value of a current to be supplied to the motor 20, based on the inputted steering torque TA and the inputted vehicle speed V. The steering assist command value I is inputted to a subtractor 30A, and is also inputted to a differential compensator 34 of a feedforward system for increasing a response speed. A difference (Ixe2x88x92i) calculated by the subtractor 30A is inputted to a proportional calculator 35, and is also inputted to an integration calculator 36 for improving the characteristic of a feedback system. Outputs from the differential compensator 34 and the integration calculator 36 are inputted to and added together by an adder 30B. A result of the addition by the adder 30B is obtained as a current control value E, and this is inputted to a motor driving circuit 37 as a motor driving signal. A motor current value xe2x80x9cixe2x80x9d of the motor 20 is detected by a motor current detecting circuit 38, and this motor current value xe2x80x9cixe2x80x9d is inputted to the subtractor 30A and is fed back.
In this case, when a driver operates the steering wheel to make a vehicle pass a curve, the steering apparatus receives force to return to a neutral point, that is, a straight-line running position, based on reactive force of the tires received from the surface of the road. Accordingly, when the driver hands off the steering wheel at the time when the car has finished passing the curve, the steering apparatus naturally returns to the neutral point based on the reactive force received from the surface of the road. As a result, the steering wheel turns to the opposite direction. This operation is generally called a xe2x80x9creturn of the steering wheelxe2x80x9d.
In general, the steering wheel of a vehicle tries to return to a neutral point based on the reactive force of the suspension called a self-aligning torque (SAT). However, in the case of the electric power steering apparatus, the rotation of the motor 20 for assist assistance is transmitted to the steering mechanism via the reduction gears 3. Therefore, there is a problem that the return of the steering wheel 1 becomes insufficient, particularly during a running at a low speed, due to the inertia moment of the motor 20 and the influence of friction of the reduction gears 3. Consequently, it is necessary to control the motor 20 to return the steering wheel 1 to a neutral point.
According to this steering wheel return control, in general, a return current that works in the steering wheel return direction is determined based on the steering angle xcex8 that is detected by the steering angle sensor 7, without making a distinction between the time of turning and the time of returning the steering wheel. However, as this current works in a direction opposite to the direction of turning the steering wheel, as a matter of fact, the driver has always felt an excessive friction and a strange touch like a spring at the time of turning the steering wheel.
Further, according to the conventional method of controlling the current for returning the steering wheel, the current is determined based on only the steering angle and the vehicle speed, without taking into account the steering angular velocity. Therefore, this gives a bad influence to the astringency of the steering wheel, at the time when the steering wheel is returned at a high steering angular velocity. Further, according to techniques disclosed in Japanese Patent Application Laid-open No. 9-277950 A and Japanese Patent Application Laid-open No. 11-34901 A, the steering wheel return control and the astringency control are switched over between them according to the vehicle speed and the steering angular velocity. However, according to the above method, there occurs time when the astringency control is not carried out during the astringent operation, and this has a risk of not exhibiting the effect of the astringency control, and giving a sense of incongruity to the driver when the controls are switched over between them.
The present invention has been made in the light of the above situations. It is an object of the present invention to provide a control unit for an electric power steering apparatus capable of improving the steering performance while taking balance between the steering wheel return control and the astringency control, by recognizing the turn and the return of the steering wheel in the steering wheel return control, and by employing the information on the steering angular velocity in the calculation of the current for the steering wheel return control.
The present invention provides a control unit for an electric power steering apparatus that controls a motor for giving steering assist force to a steering mechanism based on a current control value calculated from a steering assist command value calculated based on the steering torque generated in the steering shaft, and a current value of the motor. The object of the present invention can be achieved based on the provision of a steering wheel return controller for applying a steering wheel return control signal to the steering assist command value, thereby to execute the steering wheel return control only when the steering wheel returns.
Further, the present invention provides a control unit for an electric power steering apparatus that controls a motor for giving steering assist force to a steering mechanism, and controls the steering wheel to a neutral point by using a steering angle sensor, based on a current control value calculated from a steering assist command value calculated based on the steering torque generated in the steering shaft and a current value of the motor. The object of the present invention can be achieved based on the provision of a steering wheel return controller for applying a steering wheel return control signal to the steering assist command value, thereby to always execute the steering wheel return control and the astringency control.